This invention relates to thermal inkjet printing and, more particularly, to detecting ink flow through the printhead of a thermal printing device such as a computer printer, facsimile machine or the like.
Thermal inkjet printing is now a common method of producing high quality, low cost printing with computer printers, facsimile machines and potentially with copiers and other devices as well. The basic design and operation of inkjet printing devices are well known and amply described in U.S. Pat. No. 4,910,528, owned by the present assignee and hereby incorporated by reference. Such devices use an inkjet pen (also known as an ink cartridge), which includes an ink container and printhead through which ink from the container is ejected onto the print media.
One concern with inkjet printing is the sufficiency of ink flow to the paper or other print media. Print quality is a function of, among other things, ink flow through the printhead. Too little ink on the paper produces faded and hard-to-read printed documents. In a worst case, no ink may be printed and the entire document is lost. This scenario may occur where a facsimile machine, out of ink, receives a transmission when unattended and attempts to print. Since the inkjet pen moves across the media even when no ink is being ejected, the facsimile machine mistakenly assumes that the transmission has successfully been received and acknowledges reception to the sender.
One approach to detecting the sufficiency of ink mechanically in inkjet printing is described in U.S. Pat. No. 4,935,751, also assigned to the present assignee. The ink pen therein houses a contractible ink bag to which is attached a rigid strip. The top end of the pen housing is a window revealing the end of the strip. A scale may be attached to the window. As the ink bag depletes, it contracts and pulls the strip across the window. An observer can manually tell from the position of the strip the relative amount of ink that is left in the bag and thereby the sufficiency of ink for printing. Another mechanical technique using a ball check valve is disclosed in U.S. Pat. No. 4,940,997.
A second approach is to place a capacitive sensor on the printhead, as disclosed in U.S. Pat. No. 4,853,718. The capacitance is a function of the amount of ink present in a channel connecting the ink reservoir to the inkjet of the printhead. With ink present, a charge on the capacitor leaks off quickly. With ink absent, the charge leaks off slowly. A sampling circuit designed to measure the capacitor voltage at a certain interval detects whether there is ink in the channel. Although plausible, this approach requires the addition of relatively complex and costly circuitry to the printing device.
A third approach is to place a thermistor (a semiconductor device whose electrical resistance is dependent upon temperature) directly in the ink channel. Ink has a greater thermal conductivity than air, and the resistance of the thermistor rises as air replaces ink in the channel. The drawback of this approach is that, over time, deposits form on the thermistor which cause it to give an erroneous output. A similar technique wherein a temperature sensor is surrounded by gas or liquid is described in U.S. Pat. No. 4,326,199.
A fourth approach, shown and described in U.S. Pat. No. 5,206,668, is to compare the temperature change of the printhead at two different printing intervals to determine inkflow through the printhead. As the printhead runs out of ink, its rate of temperature change increases. By examining the ever-increasing ratio of temperature change at distinct printing intervals, this approach determines when ink flow is no longer sufficient. While accurate, this approach may occasionally provide an out-of-ink signal too late because of an anomalous reading of temperature change at one of the intervals.